Ellsworth Kelly
From Line Form Color
1951

The relationships between theory and practice are far more partial and fragmentary. On one side, a theory is always local and related to a limited field, and it is applied in another sphere, more or less distant from it. The relationship which holds in the application of a theory is never one of resemblance. Moreover, from the moment a theory moves into its proper domain, it begins to encounter obstacles, walls, and blockages which require its relay by another type of discourse (it is through this other discourse that it eventually passes to a different domain). Practice is a set of relays from one theoretical point to another, and theory is a relay from one practice to another. 

No theory can develop without eventually encountering a wall, and practice is necessary for piercing this wall.

Gilles Deleuze, Intellectuals and power: A conversation between Michel Foucault and Gilles Deleuze

Sol LeWitt
All Combinations of Arcs from Corners and Sides; Straight, Not-Straight and Broken Line
1975